The present invention relates generally to diagnostic imaging and, more particularly, to a method and apparatus for testing the low contrast performance of a diagnostic imaging system.
The x-ray dose required by a CT imaging system to produce an image is becoming an increasingly important consideration to customers and regulatory agencies. The dose efficiency of a system is commonly tested using low contrast detectability (LCD) tests. Generally, images of phantoms are used to visually test the LCD performance of an imaging system. Phantoms can simulate anatomical regions of a subject such as the heart or lungs of a subject. Other phantoms may provide a simple set of low contrast hole patterns in a uniform background. A number of phantoms are available across diagnostic modalities.
Manufacturers generally specify low contrast detectability as the smallest size hole of a given contrast that can be visualized in a given phantom at or below a specified dose. Frequently when a new system is installed, tests are performed to demonstrate system performance. Additionally, periodic quality assurance tests are often performed to assure continued proper performance. These visual LCD specifications are often difficult to objectively measure because each observer must make a claim that a hole is present or not even though holes are always present in the phantom. As a result, there is often a large variability between observers. To improve objectivity, a number of methods have been proposed including what is commonly referred to as a Four Alternative Forced Choice (FAFC) test method. With this method, twelve scans are executed of a phantom having a low contrast object randomly placed in one of four quadrants of the phantom for each scan. A skilled observer then evaluates each image and determines which quadrant has the low contrast object. Generally, to pass the test it is necessary that the object be identified in at least nine of the twelve trials. The FAFC method is considered an objective test since the claims of the observer can be objectively scored as correct or incorrect. However, the low contrast object must be repositioned for each of the twelve scans to a random quadrant location unknown to the observer. Phantoms with a large variety of low contrast test objects that can be randomly positioned for an FAFC test can be expensive and are therefore not readily available. Furthermore, the FAFC method requires a minimum of two people, one who selects the random positions and another to perform the tasks of the observer who dose not know the true location of the low contrast object. The requirement for two thereby increases the expense of performing such tests.
Therefore, it would be desirable to design an apparatus and method of objective low contrast object detection that can be implemented with a single user/observer that is also cost effective.